1. Field of the Invention
The present invention relates in general to a method of producing vibration insulators, and more particularly, to a method of producing vibration insulators used in motor vehicles or the like. More specifically, the present invention is concerned with a method of producing the vibration insulators of a type which comprises an outer cylindrical member of fiber-reinforced plastic, an inner cylindrical member of metal received in the outer cylindrical member, and a compounded rubber block interposed therebetween.
2. Description of the Prior Art
Hitherto, various types of vibration insulators have been proposed and put into practical use particularly in the field of motor vehicles. One of them is of a type which comprises an outer cylindrical member of fiber-reinforced plastic, an inner cylindrical member of metal received in the outer cylindrical member and a compounded rubber block interposed therebetween. For production of the vibration insulators of such type, two methods have been commonly used.
One method is as follows.
First, an adhesive is applied to both an inner surface of the outer cylindrical member and an outer surface of the inner cylindrical member. The adhesive is of a type which becomes activated when heated to a certain level. Then, the inner cylindrical member is coaxially received in the outer cylindrical member and they are put into a mold which has been heated to about 150.degree. C. to 170.degree. C. Then, a liquid state (or insufficiently vulcanized) rubber material is poured into the mold and thus into a cylindrical clearance defined between the inner and outer cylindrical members. For sufficiently curing the rubber material, the temperature of the mold is kept at 150.degree. C. to 170.degree. C. for a certain time. Thus, in this method, curing of the adhesive and that of the rubber material are achieved at the same time. In other words, the bonding between the outer cylindrical member and the compounded rubber block and that between the inner cylindrical member and the rubber block are achieved at the same time. For ease of understanding, this method will be named as "one-step bonding method".
The other method, which will be named as "two-step bonding method", is as follows.
First, an inner unit is produced by forming a compounded rubber block around the outer surface of the inner cylindrical member using the above-mentioned one step bonding method. That is, by this first step, the bonding between the rubber block thus formed and the inner cylindrical member is achieved. Then, the inner unit thus produced is press-fitted in the outer cylindrical member whose inner surface has been applied with an adhesive, and then, the entire structure is heated to a certain temperature to cure the adhesive. Thus, by this second step, the bonding between the compounded rubber block and the outer cylindrical member is achieved. For heating the entire structure, one method is proposed by Japanese Patent First Provisional Publication 3-221434. That is, in this method, powder of ferroelectric material, ferromagnetic material or electrically conductive material is dispersed in the adhesive, and the adhesive is heated by means of a high frequency heating.
However, due to their nature, the above-mentioned two conventional methods have drawbacks which are as follows.
That is, in the one-step bonding method, the mold is kept heated within the range from 150.degree. C. to 170.degree. C. for curing or vulcanizing the rubber material. However, such temperature range affects the mechanical strength of the plastic (viz., FRP) which constitutes the outer cylindrical member. In fact, the strength of the plastic at such high temperature becomes very low. Accordingly, there arises a possibility that the outer cylindrical member heated to such high degree is deformed and/or cracked due to a marked pressure produced by the rubber material poured into and cured in the mold. Furthermore, if the outer and inner cylindrical members are different in length, the length of the mold must be matched with the longer one. This however induces an ill-matched increase in weight of the mold.
In the two-step bonding method, the inner unit is press-fitted in the outer cylindrical member applying a marked stress to the outer cylindrical member. The press-fitting however affects the heat resistance of the outer cylindrical member constructed of FRP. That is, due to a marked stress applied to the outer cylindrical member, the heat resistance of the same is lowered to the level of 100.degree. C. to 120.degree. C. which is very low as compared with the activation temperature (viz., 130.degree. C. to 200.degree. C.) of the adhesive. Of course, the temperature suitable for the outer cylindrical member and the temperature suitable for the adhesive are not obtained by means of a simple heating method. If such simple heating method is actually used, deformation and/or poor bonding of the outer cylindrical member tends to occur. This drawback may be solved by using the method disclosed by the above-mentioned Japanese Patent Publication 3-221434. However, in this method, a new problem arises in that the powder (viz., powder of ferroelectric material, ferromagnetic material or electrically conductive material) dispersed in the adhesive has a bad influence on the essential work (viz., bonding) of the adhesive. Furthermore, when the amount of the powder in the adhesive increases, it becomes necessary to make a frequent stirring of the adhesive before and during application of the powder-mixed adhesive to the outer cylindrical member.
It is said that when a compounded rubber is mainly composed of polar rubbers whose basic polymer or polymers have SP value (viz., solubility parameter) larger than 8.6, a partial heating of the rubber is possible by means of the high frequency heating. Thus, in this case, activation of an adhesive contacting the heated part of the rubber may be expected. However, in general, the compounded rubbers having such polar rubbers fail to have a satisfied vibration insulation ability. Furthermore, since such rubbers are relatively expensive, the vibration insulators including them would have a poor competitive position in markets even though the bonding by the adhesive is made well.